Every year, the Earth is hit by about 6100 meteors large enough to reach the ground, or about 17 every day, research has revealed.
The vast majority fall unnoticed, in uninhabited areas. But several times a year, a few land in places that catch more attention.
Three months ago, for example, a small asteroid probably about the size of a minivan, flashed across the midday sky and exploded over western Cuba, showering the town of Viñales with falling rocks, some of which reportedly landed on rooftops.
Nobody was hurt, but it was a reminder that just as it’s not safe to turn your back on the ocean for fear of being washed out to sea by an unusually large wave, space hazards are also capable of catching us by surprise.
To calculate how often such meteor falls occur, Gonzalo Tancredi, an astronomer at the University of the Republic in Montevideo, Uruguay, examined a database of incident reports, discovering that in the last 95 years people have directly observed 95 such events – an average of about eight per year.
To figure out how many others occur unobserved, Tancredi noted that people only occupy a tiny fraction of the Earth’s surface — about 0.44% of its land area, or 0.13% of its total surface area.
That means that for every impact that is actually seen by someone, another 770 splash into the sea or fall in a desert, forest, or other locations so remote that nobody sees it happen.
“Some places on the Earth are heavily populated,” Tancredi says, “but most places are very lowly populated.”
Tancredi then got to wondering if such impacts might occur in swarms. He wondered, in fact, whether there is a time of year at which the chances of being hit by a falling rock — however low they are, on average — might be elevated.
It’s not a crazy idea. We know, for example, that the smallest meteors — far too tiny to reach the ground — often come during meteor showers, such as the Perseids, Orionids, and Gemonids, which enchant sky-watchers several times a year.
Could metre-sized rocks, large enough for their fragments to reach the ground, also be lurking in these annual events or be otherwise happening on a recurring basis?
The answer, Tancredi reported recently at the International Academy of Astronautics’ Planetary Defence Conference (IAAPDC) in College Park, Maryland, US, is a definite no.
“The falls are random throughout the year,” he says. “There is no particular date of the year to be outdoors with a helmet.”
Not that being bonked on the head by rocks the size of baseballs is the only way space can sneak up on us.
Scientists claim to have mapped out the orbits of the vast majority of potential “dinosaur killer” asteroids in near-Earth orbit.
However, Duncan Steel, a space scientist who lives in Wellington, New Zealand, but works for the NASA-Ames Research Centre in California, told the IAAPDC the assertion was “a little bit of a fib”.
“In terms of dino-killers we’ve only found a tiny fraction,” he said.
The reason, he adds, is because an unknown number of potentially dangerous asteroids are not in near-Earth orbits. Rather, they are in elongated orbits that loop far out into the outer solar system then dive back toward Earth, which they might someday hit.
“Most are in these giant orbits [where] we aren’t going to find them on a 20-year or 50-year time frame,” he explains.
That said, only 66 such asteroids have yet been found, and each has only a one-in-two-billion to three-billion chance of hitting the planet on any of its five-to-20-year passages around the sun.
“How large [an] impact threat that represents, depends on the size of the [asteroid] population,” Steel adds.
An even harder to calculate risk, said Yudish Ramanjooloo, a near-Earth objects postdoctoral fellow at the University of Hawaii, Honolulu, comes from a recently discovered class of comets known as Manx comets.
Named for Manx cats, these are comets so inactive that they do not produce visible tails. In fact, Ramanjooloo says, their activity is five to six orders of magnitude lower than that of typical comets.
Like normal comets, however, they dive deep into the inner solar system from origins well beyond the orbit of Pluto, and possibly close to the boundary of interstellar space.
That means that when they reach us, they are coming in hard and fast. Their lack of tail-forming volatiles also means they are rocky and dense, capable of hitting with enormous amounts of energy.
Worse, the lack of tails makes them hard to spot until they are practically upon us. If you are looking for a disaster-movie scenario, a Manx comet, not seen until less than a month before impact, might be as good as it gets.
“I think the Manxes will have very short warning times and will impact us hard,” Ramanjooloo says.
Perhaps it is they, not falling rubble like that which rattled down on Viñales, that are the true sneaker waves of outer space.
Richard A Lovett
Richard A Lovett is a Portland, Oregon-based science writer and science fiction author. He is a frequent contributor to Cosmos.
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